1. Field of the Invention
This invention relates to lamps or light sources. More particularly, a light source having a substantially planar cold-emission cathode as a source of electrons and a pulsed electrical potential between the cathode and a phosphor-coated anode is provided.
2. Description of the Related Art
The efficiency of electric lamps using filaments has not increased significantly for many years. Fluorescent discharge lamps are more efficient, but have other limitations which have been difficult to remove.
Cathodoluminescent lamps have been known for several years. They originally employed thermionic cathodes. U.S. Pat. No. 4,818,914 provided an improvement in cathodoluminescent lamps based on a field emission cathode. A layer of phosphor on an anode is located inside an envelope along with the field emission cathode, which is placed opposite the phosphor layer. A voltage source, either DC or AC, is connected across the cathode and accelerator electrodes to cause field emission of electrons from tips of needle-like members. A higher voltage is connected across the cathode and an anode for attracting electrons to the phosphor layer and exciting the phosphor to luminescence.
A thin-film-edge field emitter in a two-dimensional array of microstructures placed opposite a cathodoluminescent phosphor screen has been suggested as a backlight in Active Matrix Liquid Crystal Displays (AMLCD) (xe2x80x9cThin-film-edge emitter vacuum microelectronics devices for lamp/backlight applications,xe2x80x9d Eighth International Vacuum Electronics Conference, Portland, Jul. 30, 1995, p. 418-422.) It is suggested that such field emitter arrays can potentially satisfy the brightness, power efficiency and dimability requirements for AMLCD backlights. Such emitters would be used with high voltage phosphors so as to achieve the high brightness and long phosphor lifetime needed.
Backlighting is generally necessary in all Liquid Crystal Displays (LCDs). Such displays are used in televisions, lap-top computers, and various types of aircraft and automotive displays. The backlight in these systems is normally provided from a miniature fluorescent lamp. In many of these applications the backlight is the largest single consumer of power in the display system. For example, in a lap-top computer, the backlight will consume approximately 40 percent of the total power and the fluorescent lamp providing the backlight consumes a large fraction of that power. There is clearly a need for high efficiency lamps for such applications. In aviation and automobile displays, there is a need for a light source that can be dimmed and that will operate at low temperature without requiring auxiliary heat, that has quick startup, and that does not fail catastrophically. For other types of displays and light sources, such as projectors, a high brightness is needed. In some applications it is important to have the capability to scale-up the size of the source to provide the total light output needed. In all light sources, there is a need for a simple and efficient source and a source that does not cause potential environmental pollution, such as from the mercury that is present in fluorescent tubes. As in any device, a long lifetime is also desired.
A cathodoluminescent lamp is provided having a flat cold cathode. The cathode has a low effective field emission work function. In one embodiment, the cathode surface is made of a carbonaceous coating which may be formed by laser ablation. In other embodiements, the cathode is formed by chemical vapor deposition or other methods. In a preferred embodiment, high voltage pulses having a low duty cycle are used to create the electric field between the cathode and an anode having phosphors adjacent to a transparent electrical conductor. In other embodiments, the cathode is comprised of separate areas having low effective field emission work function or the anode is comprised of separate areas of a a phosphor-coated anode. Mixtures of phosphors may be used.
A method for forming a lamp is disclosed. The method includes supplying a transparent anode coated with phosphor and a flat cathode having low effective field emission work function and encapsulating the electrodes and phosphor in an evacuated envelope. High voltage, by, preferably in the form of pulses, is then supplied between the electrodes.